Best way to do this is to play Hotzone, find one of the special weapon crates on the map and hope to get the Airstrike canister. The weapons in the crates are random so this may take a few tries. One you get the airstrike, run toward the objective and wait for the opposing team to arrive and toss the canister. It has a rather large explosion radius so anyone in the area will get hit.
Also try not to die before throwing the airstrike, you lose special weapons if you're killed.
Alternate method, start up a private match with enough people and kill 4 of them with any explosive. (Grenade, Rockets, Grenade Launcher, Mine, Airstrike)